With advancements in technologies, the demand on high signal transmission speed has become increasingly more. Currently, a high speed signal is usually transmitted through a differential transmission in high speed. The so-called differential transmission refers to a method for making current flow inversely in two transmission lines while conducting transmission with use of a potential difference between the transmission lines, which can provide the advantage of strong anti-interference capability.
FIG. 1 is a schematic partial view of a conventional multi-layer circuit structure. FIG. 2 is a schematic partial top view of the multi-layer circuit structure of FIG. 1. It should be noted that, FIG. 1 merely illustrates a part of differential transmission related circuits for simplicity of drawing, in which dielectric layers or other circuits among circuits on different planes are omitted. Further, because the differential transmission related circuits are located on the different planes, the circuits located on a lower plane are illustrated by dotted lines in the top view of FIG. 2, so the circuits on the different planes may be distinguished.
Referring to FIG. 1 and FIG. 2, a conventional multi-layer circuit structure 10 includes two differential transmission line pairs 11a and 11b, two pairs of differential pads 18a and 18b, and two plated through vias 19 and 20. The two differential transmission line pairs 11a and 11b are located on different planes and electrically connected to each other through the differential pads 18a and 18b and the plated through vias 19 and 20.
As shown in FIG. 1 and FIG. 2, the differential transmission line pair 11a includes a first transmission line 12a and a second transmission line 15a disposed side by side. The first transmission line 12a includes a first segment 13a and a second segment 14a connected to each other, and the second transmission line 15a includes a first segment 16a and a second segment 17a connected to each other. The differential transmission line pair 11b includes a first transmission line 12b and a second transmission line 15b disposed side by side. The first transmission line 12b includes a first segment 13b and a second segment 14b connected to each other, and the second transmission line 15b includes a first segment 16b and a second segment 17b connected to each other.
As restricted by the difference of the minimum spacings S2, S4 and the minimum interval D in production of the circuit and the plated through vias, an spacing S1 between the two first segments 13a and 16a is non-fixed and an spacing S2 between the two second segments 14a and 17a is fixed in the differential transmission line pair 11a so the circuit layout of Y-junction is formed, for example. A spacing S3 between the two first segments 13b and 16b is non-fixed and a spacing S4 between the two second segments 14b and 17b is fixed in the differential transmission line pair 11b so the circuit layout of Y-junction is formed, for example. A minimum distance (minimum interval) D between the two plated through vias 19 and 20 is normally different from (e.g., greater than) a spacing S2 between the second segment 14a of the first transmission line 12a and the second segment 17a of the second transmission line 15a in the differential transmission line pair 11a. The minimum distance D between the two plated through vias 19 and 20 is normally different from (e.g., greater than) a spacing S4 between the second segment 14b of the first transmission line 12b and the second segment 17b of the second transmission line 15b in the differential transmission line pair lib. However, because the non-fixed spacings S1, S3 of segments exist between the first transmission line 12a and the second transmission line 15a and between the first transmission line 12b and the second transmission line 15b, the impedance changes as the spacing becomes different. Accordingly, signal transmission loss will increase to lower signal transmission quality.